Steam turbine airfoils or buckets generally are positioned about a rotor at regular intervals in a bucket assembly. The bucket assembly may be created by inserting the buckets one at a time tangentially into an opening on the rotor and then sliding the buckets circumferentially about the rotor. The buckets may be attached to the rotor by complementary male and female dovetails and other configurations. In order to close the bucket assembly, however, the last bucket must be restrained by a feature other than a dovetail. This last bucket, generally called the locking blade or the closure bucket, may be affixed to the rotor via one or more blade retention screws and the like tapped or screwed into the rotor. Other types of connection means and other types of bucket assemblies also may be used.
Large centrifugal loads may be placed on the buckets and the rotor during operation. Such centrifugal loads and coincident thermally induced loads associated with loading transients may induce stresses in the dovetails and adjacent areas that attach the buckets to the rotor. These stresses may be of sufficient magnitude to impact adversely rotor cycle fatigue life. Of particular concern may be rotor stress concentrations associated with blade retention screws that may be tapped or otherwise inserted directly into the rotor.
There is thus a desire for an improved turbine rotor and blade assembly for a steam turbine and the like. Preferably such an improved turbine rotor and blade assembly may reduce stress concentrations therein for an improved overall rotor fatigue life.